It has previously been proposed to make pressure transducers in which capacitive plates are spaced apart and mounted on one or more ceramic diaphragms. Such a transducer is disclosed, for example, in U.S. Pat. No. 4,388,668 (the '668 patent) assigned to the assignee of the present invention. As disclosed in the '668 patent, one plate of a capacitor may be mounted on a flexible circular ceramic diaphragm made of alumina, and the other plate of the capacitor may be mounted in a facing relationship on a heavier substrate. The diaphragm may be spaced from the substrate around its periphery by the use of a glass frit, which not only spaces the diaphragm apart from the substrate but also seals the periphery of the diaphragm to the substrate. When pressure is applied to the outer surface of the diaphragm, it deflects toward the substrate and the two facing conductive plates come closer together. When the distance between the two capacitive plates changes, the capacitance between the plates changes. The changing capacitance can be detected by an electronic circuit as an indication of the pressure applied to the diaphragm.
The assignee of the present invention produces a full line of pressure transducers which utilize alumina ceramic diaphragms. Many of these transducers are patented. Many of the assignees transducers have certain similarities which allow for standardization of parts and manufacturing procedures across the whole line of transducers which the assignee produces.
For example, U.S. Pat. No. 4,227,419 issued Oct. 14, 1980 and assigned to the assignee of the present invention, illustrates one type of transducer produced by the assignee. The '419 patent shows a transducer with a diaphragm preferably made of alumina and also illustrates the circuit used to detect the changes in capacitance which indicate the pressure applied to the alumina diaphragm. U.S. Pat. No. 4,398,426 issued Aug. 16, 1983 to Park et al. (the '426 patent) also assigned to the assignee of the present invention, shows another transducer also produced by the assignee of the present invention. The '426 patent shows a circuit which improves the linearity of the output voltage versus pressure characteristic of the transducer.
A capacitive pressure transducer such as those disclosed in the '668 patent, the '426 patent and the '426 patent can be made with ceramic diaphragms as thin as 0.010 inches. However, if the ceramic diaphragm is made any thinner, it will crack. Transducers made with the thinnest possible ceramic diaphragms have a pressure range at full scale of approximately 5 psi (where one atmosphere is about 14.2 psi). Therefore, capacitive pressure transducers made with ceramic diaphragms are not useful in measuring pressure differences where the full scale range should be substantially less than 5 psi. In an application such as air conditioning system pressure measurements, for example, it would be useful to take measurements in the range of 0.18 psi.
Previously issued patents for low pressure transducers disclose transducers which are manufactured or shaped such that they are not compatible with the product line of the assignee of the present invention. For example, U.S. Pat. No. 4,606,228 issued Aug. 19, 1986 to Whitmore (the '228 patent) discloses a low pressure transducer which utilizes a diaphragm composed of a perforated metallic disc overlaid with a thin plastic film. The transducer assembly disclosed in the '228 patent, shown in FIG. 3, is completely incompatible with the assembly methods and product line of the assignee of the present invention. Furthermore, the diaphragm utilized in the transducer disclosed in the '228 patent is more expensive and difficult to manufacture.
Another low pressure transducer is disclosed in U.S. Pat. No. 4,358,814 issued Nov. 9, 1982 to Lee et al. (the '814 patent). The '814 patent discloses a transducer which uses a conductive deformable diaphragm which is stretched across a flange and held by a clamping ring. Manufacturing a transducer with such a diaphragm is also difficult and expensive. U.S. Pat. No. 4,785,669 issued Nov. 22, 1988, discloses a single fluid-pressure transducer unit which utilizes a sheet metal diaphragm and an elastically deformable electrode. U.S. Pat. No. 2,999,386 issued Sep. 12, 1961, discloses a thin deformable diaphragm which is under substantial radial tension. U.S. Pat. No. 4,466,289 issued Aug. 21, 1984, discloses a transducer which utilizes a metal diaphragm under uniform tension, which would appear to have relatively low capacitance and to be non-linear. Furthermore, the geometry of such a transducer system is totally incompatible with the product line of the assignee of the present invention.
Therefore, what is needed is a capacitive pressure transducer which utilizes a diaphragm which is sensitive to very slight pressure changes, is relatively inexpensive and simple to manufacture, is of a similar shape and geometry as the assignee's alumina disk diaphragm transducers and is therefore compatible with the product line of the assignee of the present invention. Such a transducer could utilize the electronic circuits disclosed in the '426 patent and the '419 patent.